High performance, high speed diesel engines are often equipped with turbochargers to increase power density over a wider engine operating range, and EGR systems to reduce the production of NOx emissions.
Turbochargers use a portion of the exhaust gas energy to increase the mass of the air charge delivered to the engine combustion chambers. The larger mass of air can be burned with a larger quantity of fuel, thereby resulting in increased power and torque as compared to naturally aspirated engines.
A typical turbocharger consists of a compressor and turbine coupled by a common shaft. The exhaust gas drives the turbine which drives the compressor which, in turn, compresses ambient air and directs it into the intake manifold. Variable geometry turbochargers (VGT) allow the intake airflow to be optimized over a range of engine speeds. This is accomplished by changing the angle of the inlet guide vanes on the turbine stator. An optimal position for the inlet guide vanes is determined from a combination of desired torque response, fuel economy, and emissions requirements.
EGR systems are used to reduce NOx emissions by increasing the dilution fraction in the intake manifold. EGR is typically accomplished with an EGR valve that connects the intake manifold and the exhaust manifold. In the cylinders, the recirculated exhaust gas acts as an inert gas, thus lowering the flame and in-cylinder gas temperature and, hence, decreasing the formation of NOx. On the other hand, the recirculated exhaust gas displaces fresh air and reduces the air-to-fuel ratio of the in-cylinder mixture.
Both the VGT and EGR regulate gas flow from the exhaust manifold, and their effect is, therefore, coupled through the conditions in the exhaust manifold. Excessive EGR rates displace the intake of fresh air and may lead to incomplete combustion of the injected fuel which, in turn, could cause visible levels of smoke and increased levels of emissions. Moreover, this could negatively affect fuel economy and/or performance. Thus, for effective control of diesel engines with EGR systems, it is necessary to control the EGR flow precisely, not only in steady state but also in transient conditions. Precise EGR control requires monitoring the EGR flow, the fresh air flow, and the intake mixture composition to control the combustion process and, thereby, avoid smoke production and particulate emissions.
Conventionally, fuel demand is calculated as a function of engine speed and accelerator pedal position, and fueling rate limiters restrict the actual amount of fuel injected as a calibrated function of intake manifold pressure (MAP), intake manifold temperature, and engine speed (RPM) to avoid smoke. Thus, such systems operate without regard to measurements or estimates of the amount of fresh air or oxygen available in the intake manifold. In particular, fuel limiting is conventionally accomplished as a function of charge density, without regard to charge composition, or as a function of compressor mass airflow, without regard to the additional oxygen contributed by the EGR flow.
Knowledge of the amount of fresh air available for combustion in the intake manifold, however, is important. In a diesel engine, the generated torque is directly proportional to the amount of injected fuel, provided enough air is available. Visible smoke is also strongly related to the air/fuel ratio. Consequently, it is desirable to deliver fuel to the engine at a rate which generates the torque demanded by the driver, yet maintains the air/fuel ratio above the threshold at which visible smoke occurs.